After Mymble's family comes to visit the Moomins, her littlest and most badly behaved sister, Little My, is left behind. She promises to behave if she is given Moomin's bedroom; Moomin, tired of being forced out of his room, decides to build a house where all the rooms will belong to him. With Little My helping out, things are sure to go awry, but in the end, Moomin's house-building misadventures teach him the value of a home.
